Wireless communication systems are divided into a number of geographic sectors. At least one base station is associated with a sector. Mobile stations within the sector can communicate with other mobile stations, within the same or a different sector, or with external networks, via the base station. At any given time, each mobile station typically communicates with only a single base station, however the base station must be able to communicate with all of the mobile stations within its sector.
Some base stations employ a scheduler to allocate resources among their serving mobile stations. One type of scheduler that may be used is referred to as a proportional fair scheduler. This scheduler is designed to balance fairness of service among the mobile stations against the maximization of overall throughput of data by the base station.
The base station scheduler typically operates on data rate requests from the mobile stations within the sector. The rate at which data can be reliably received at a given mobile station is determined by the signal-to-interference-and-noise ratio (SINR) of the signal received by that mobile station. A higher SINR level provides greater reliability and allows for a higher data rate. At lower SINR levels, transmissions are less reliable and lower data rates are used. Since mobile stations' SINR levels generally fluctuate, the supportable data rates also fluctuate.
The proportional fair scheduling algorithm accomplishes scheduling decisions by comparing the ratios of the requested data rates to the average throughputs of each mobile station. As a result of the SINR fluctuations seen by each mobile station, the scheduler tends to distribute service among the mobile stations, each at its peak data rate. Consequently, the scheduler achieves what is referred to as multi-user diversity gain. Multi-user diversity gain generally increases when the number of mobile stations in the sector increases, and when the dynamic range of SINR fluctuations seen by the mobile stations increase.
The SINR level at each mobile station may fluctuate for a number of reasons, including multipath fading and movement within a sector. There are also ways to increase the SINR fluctuations artificially. One way to do this is to use a certain type of forward power control (FPC) scheme that induces SINR fluctuations for each mobile station. In one such scheme, the base station transmits at full power through an antenna array. The phase of the signal fed to each array element is varied as a function of time, in a periodic manner. The resulting beam pattern sweeps through the sector periodically. As the beam sweeps across a particular mobile station, the SINR level for that mobile station peaks and then falls off according to the periodic variation.
A mobile station uses a SINR measurement to request the highest data rate from the base station that it can decode with low probability of error. Since the SINR level typically fluctuates, the mobile station can employ a predictor to forecast the average SINR of the next packet in time so as to avoid requesting too low or too high of a data rate. However, when the SINR fluctuations become too large, or the SINR varies too quickly, even the use of a predictor can lead to a requested data rate, which will either underutilize base station transmissions or otherwise lead to error.